Mineralogy and Geochemistry of Eteké Eburnean Gold Deposit (Gabon) ()
1. Introduction
1.1. Geology of Gabon
The examination of the Gabon map helps to differentiate four major tectonostratigraphic sets [1] designated in a regional context. From the bottom to the top, we found the Archean basement, the Paleoproterozoic formations, the Neoproterozoic of Nyanga, and the Phanerozoic groups (Figure 1):
· The Archean domain, part of the Congo craton, extends from Congo to Cameroon crossing Gabon and represent the Precambrian basement of the central Africa [2] [3]. It consists of Archean cratons welded together by the Mesoproterozoic and Paleoproterozoic belts [4] [5] and represented mainly, in the Gabon territory, by the Chaillu massif and the North Gabon massif. Its establishment is the result of significant magmatic activities that led to the formation of diverse ultrabasic and granitoid rocks that are the subject of several datations in the Chaillu massif [1] [6] [7] [8].
![]()
Figure 1. Simplified geological map of Gabon, defining the main structural domains (modified [1]).
· The Paleoproterozoic domain is the result of the collision between the Congo and Sao Francisco cartons during the Eburnean orogeny (2500 - 2000 Ma) [9] [10] [11]. In Cameroon, Central Africa, Congo and more particularly Gabon, the Eburnean orogeny has been recognized by a folded belt similar to the Phanerozoic belt dominated by modern plate tectonics [1] [12] [13]. In the central part of Gabon, this belt is characterized by the Ogooué mobile zone and particularly by the establishment of intracratonic basins in the Francevillian such as the Booué, Lastourville, Okouja and Franceville basins (Figure 1) [10] [12] [14] [15] [16] [17]. These basins lay in discordance on the Archean base of the Chaillu massif. The well-known basin is the Franceville basin, since it contains manganese deposits [18] [19] [20] [21] and uranium [15] [16] [22].
· The Neoproterozoic domain includes geological formations represented by the Mayombe-African belt which dominates in the south of the country. This belt is formed by massive Paleoprotezoic granite covering the Neoprotezoic deposits of the Pan-African foreland basin [23].
· The Phanerozoic domain outcrops in the West at the location of the coastal basin linked to the opening event of the South Atlantic Ocean during the Lower Cretaceous [24] and in the East at the location of the Paleogene to Quaternary sandstones and the aeolian sands of the Batéké plateau [8] [25].
1.2. Geological Situation, Location and Description of the Eteké Gold Showings
From 1937 to 1959, the Eteké gold district, located in southern Gabon, was the primary source of gold production in the country. Several tons of alluvial gold were extracted [11] mostly in the Dando-Mobi sectors (ore of 2.3 - 7.2 t grading 15 g/t), Ovala (ore 2 t grading 11 g/t) and Dango (5.8 t grading 8.4 g/t and ore 8.4 grading 2.4 - 9.8 g/t). This corresponds to gold veins set up at the time of the Paleoproterozoic Eburnean orogeny between 2.0 and 2.2 Ga [26] and hosted in the greenstone belts of Archean ages of the Chaillu massif (Congo Craton) [1] [27]. These green rocks are essentially formed from amphibolites traversed by granitoid intrusions [1] [28].
In the Eteké region, the gold mineralization is linked to the Eburnean orogeny and it is the most interesting in this deposit category. Five primary sites have been recognized in the Etéké region (Figure 2) and will be the subject of our study:
· Dondo-Mobi: Prospect essentially hosted in ultrabasic rocks of the Etéké group, located between the thrust plane and the Staurolite mica schists of the Ogooué supergroup [28].
· Dango: The supporting bedrock of the Dango deposit is mainly composed of sedimentary rocks from the Massima supergroup [28]. The lithological terms are represented by black muscovite-chlorite schists and jasper recrystallized into massive or banded quartzites.
· Ovala: District located in a Paleoproterozoic syncline, wedged between two
![]()
Figure 2. ((a) and (b)) Example of SIM affecting the tectonic fabric; (c) Opening and displacement of the expansion zone filled with iron carbonates; ((d) and (e)) Quartz veins intersecting the SIM silicified foliation; ((f) and (g)) Mineralized zones characterized by SIM with cubic pyrites; (h) Brechiated zone superimposed on the SIM with cement hematite.
domes of remobilized Archean migmatites [28]. This synclinal filling is filled with sediments and lavas from the Etéké group, the Massima and Ogooué supergroups.
2. Material and Methods
The Etéké gold district is characterized by significant vegetation cover and a well-extended lateritic alteration profile. Core drillings data were therefore of paramount input to our study, as they enabled us to define the relationships between host rocks and mineralized veins.
Therefore, a total of 700 rock samples from 106 core holes from the study area were used to make the thin sections, the polished section and to perform the geochemical analyzes. Petrographic and metallogenic examinations were carried out by the optical microscope and the metallographic microscope within the Faculty of Sciences Semlalia University Cadi Ayyad Marrakech, and Faculty of Sciences Ben M'sik University Hassan II Casablanca, in order to define the mineral phases as well as the metallic phases and determine their relationship with the surrounding area. Also, analyses on total rocks in major and trace elements by ICP-AES and ICP-MS were respectively carried out at the REMINEX research center and laboratory in Marrakech, respectively, using a Horiba Jobin Yvon ULTIMA 2c and Thermo-fisher X series 2 spectrometers.
3. Results and Discussion
3.1. Description of Eteké Gold Prospects
3.1.1. Dango Zone
The Dango zone corresponds to a zone with significant, massive and whitish silicification (Figure 2(a), Figure 2(b)) (SIM: silicification, intense and massive). The development of this SIM is synchronous with the deformation, on the basis that the tectonic fabric is permeated in the silicification.
The silica impregnation is recognized, which generates textures in flames or lenses. The red-orange hues are largely related to iron carbonates and fill the compatible working thickness with a shear play indicative of tectonic movement during the silicification and iron carbonate impregnation (Figure 2(c)). The quartz veins which intersect the silicification suggest a genetic link with the main mineralization zone (Figure 2(d), Figure 2(e)). The mineralization takes the form of total silicification, whitish with cubic pyrites (Figure 2(f), Figure 2(g)). It overlaps with an episode of SIM breccia expressed by fine cement hematitization (Figure 2(h)).
3.1.2. Dondo-Mobi Zone
The Dondo-Mobi deposit has been known for its mineralization associated with hydrothermal alteration by the biotite and quartz veins. The metamorphosed volcano-sedimentary host is relatively homogeneous with a microgranular texture with andesitic compositions (Figure 3(a), Figure 3(c)). Here, the deformation is well defined and expressed by an alignment of metamorphic biotite. Locally, the silicification of the host is accentuated in the vicinity of the tourmaline-pyrite zones (Figure 3(b)) ant the hydrothermal alteration is manifested by the random distribution of biotites in the foliation, indicating low foreshortening deformation under high fluid pressure (Figure 3(d)).
3.1.3. Ovala Zone
The Ovala deposit has been characterized for its dominant clastic sedimentary rocks, highly developed muscovite schists, extensive silicification, and kyanite porphyroblasts. Here, Muscovite schist is the most dominant feature, but represents an epiphenomenon to mineralization. The muscovite is nested within each other, revealing no deformation (Figure 4(a)) and clearly intersecting pyrites (Figure 4(b)). The presence of kyanite porphyroblast indicates a superimposed metamorphism on bleached rocks. This is a typical feature of metamorphosed volcanogenic deposits into amphibolite facies (Figure 4(c)).
![]()
Figure 3. (a) Massive appearance of andesites; (b) Silicified zone within andesites, with quartz passages, tourmaline and pyrite development/Different manifestations of biotite (c) in impregnation of volcano-sedimentary; (d) in impregnation in schistosity planes.
![]()
Figure 4. (a) Interconnected muscovites without evidence of deformation; (b) Muscovite schist clearly intersecting pyrites; (c) Kyanite porphyroblasts; (d) is developed on finely bedded detrital sedimentary rocks.
3.1.4. Massima Zone
The studied cores in the Massima district expose a volcano-sedimentary sequence (Figure 5(a)) injected into mafic sills. These rocks are weakly deformed, except in the graphitic shales zones they present a very strong deformation (Figure 5(b)). These boreholes contain several gold zones closely related to disseminated pyrite in the silicified volcano-sedimentary zones (Figure 5(c)).
3.1.5. Moukanda Zone
In this study area, the treated boreholes display gabbroic sequences with particle size varying from microgranular to granular. The invasion of the structural fabric seems to have a link with the grain size: the less grainy parts are the least affected by the deformation. Probably, it could be due to the metamorphic recrystallization.
3.2. Petrographic and Metallographic Characterizations of the Study Area
The Etéké Gold deposit represents a wide facies variety, it is encased in granitoids. These rocks have been located in the base of the Chaillu massif, and characterized with a magmatic texture that are not sufficiently preserved and affected by metamorphism. The samples prepared from the core’s boreholes allowed us to determine the different facies below:
3.2.1. Sandy Chloritized Metapelite
It is a very thin facies composed of micaceous and sandy levels. These levels are composed of very fine crystals of quartz, intensely recrystallized and flattened according to the regional schistosity (Figure 6(a)). Some white micas also appear
![]()
Figure 5. (a) Volcano-sedimentary sequence; (b) well-deformed graphitic shales with folding and boudinage of siliceous ribbons; (c) Ribbons of pyrite (Py) developed along the bedding.
![]()
Figure 6. (a) Microscopic bservation of sandstone levels (LP); (b) Micaceous fragment in the pelitic level (LN); (c) Dissemination of chalcopyrite in the sandstone level (LN); (d) Silver grain (LN).
in these levels.
The micaceous levels are dominated by sericite and muscovite. These micas are generally oriented according to the regional schistosity (Figure 6(b)). In the vicinity, the white micas are strongly folded and attest the presence of a late deformation phase. Also, there is a strong chloritization of the rock which is manifested by a development of chlorite instead of the white micas. Opaque minerals are very abundant in this facies, giving it a reddish appearance.
From a metallogenic point, this facies contains chalcopyrite in ranges (50 to 100 μm), disseminated (Figure 6(c)), some very fine grains of arsenopyrite (<50 μm) and very fine droplets (<5 μm) of silver (Figure 6(d)). We also note the presence of grains and granules of iron oxides (20 - 200 μm) strongly disseminated in the rock.
3.2.2. Rubefied Quartzite
This facies have gathered mainly intense recrystallized quartz. The mineralogy observed indicates that the fragments of the studied rock would correspond to a quartzite. Nevertheless, some fragments with the same mineralogy but their particle size is coarser would correspond to quartz veins or faults that have been deformed in the same way as quartzite (Figure 7(a)). The rock cement is siliceous, more or less reddened (Figure 7(b), Figure 7(c)).
Indeed, it is strongly impregnated with iron oxides which has been invaded in
![]()
Figure 7. (a) reddened quartzite and quartz veins (LN); (b) Microscopic appearance of quartz crystals with oxides cement (LN); (c) Mass of oxids observed in the facies (LN); (d) Golden grain (Au natif) in association with iron oxides (LN).
the fragments by the fractures and cracks that affect them. This facies are characterized by a strong impregnation of iron oxides in the form of micrometric to millimeter ranges covering the entire rock (Figure 7(d)).
3.2.3. Meta-Andesite
This rock shows a microlithic porphyry texture (Figure 8(a), Figure 8(b)) whose mineralogy is formed by:
· Quartz in the form of crystals, recrystallized and forms the cryptocrystalline matrix of the rock.
· The Feldspar, which is more or less abundant in these facies, is corroded at the edge. Its peripheries are frequently confused with the quartzo-feldspar matrix. Its size varies from 200 to 500 μm. It is usually very altered with a characteristic dirty appearance (Figure 8(c)).
· The weathering minerals observed are quartz, chlorite and white micas.
· The Plagioclase is relatively abundant in these facies. It is in the form of crystals of variable size (a few microns to 1 mm). They often have polysynthetic twin, and sometimes are strongly altered in chlorite, calcite, and epidote.
· The amphibole, dark green to light in color, is either in the form of ranges, micrometric to millimeter dimension, with cross-sections and longitudinals of greater dimension, or in the form of clusters with several combined ranges (Figure 8(a), Figure 8(c)).
The cryptocrystalline matrix of the rock is represented by very chloritized quartz-feldspar nature. This chloritization, giving the rock a characteristic
![]()
Figure 8. (a) Meta-andesite (LP); (b) Amphibole observed in meta-andesite (LN); (c) Amphibole sections observed in meta-andesite (LP); (d) Microscopic appearance of sulfo -quartz vein (LP); (e) Microscopic view of pyrite in meta-andesite (LN); (f) Fine inclusions of chalcopyrite within pyrites (LN).
greenish appearance, is interpreted by a massive development of chlorite, in isolated or massive flakes developing on biotites, amphibole and on the matrix. In addition, we still note the presence of the epidote in finely disseminated granules, sometimes grouped. Carbonates completely invade the matrix, attesting to a late carbonation (Figure 8(d)).
From a Metallography view, the pyrite is very abundant in this facies, in the form of isolated crystals (50 to 500 μm) automorphic to xenomorphs finely disseminated in the rock; or grouped into several individuals, sometimes forming masses exceeding 1 mm (Figure 8(e)). These pyrite masses, in combination with quartz and carbonates, fill the fractures affecting the rock. Chalcopyrite has also been observed in this facies. It is in the form of ranges of dimensions from 50 to 200 μm, associated with pyrite and quartz at fractures, affecting the rock. Locally, it appears either in the form of scattered beaches; or in very fine inclusions within pyrite (Figure 8(f)).
3.2.4. Amphibolite
It is a deformed, recrystallized facies slightly oriented according to the direction of foliation. It is recognized by the following mineralogy: amphibole, biotite, chlorite, calcite and quartz. The Amphiboles are very common (Figure 9(a)), they are in the form of elongated crystals according to the direction of foliation. Some crystals have a Poikilitic texture. Chlorites exhibit a radio-fibrous form (Figure 9(b)) and biotites present in a fibrous aspect are also visible in the rock (Figure 9(b)).
In these facies, the Quartz, slightly present, are recrystallized forming triple points with rolling extinction and corroded by the matrix of the rock. However, Calcite is abundant and permeates the entire rock (Figure 9(c)). Under a metallographic microscope, this structure shows the presence of very fine iron hydroxide boundaries in the late cracks and fractures and very fine grains (<5 μm) of Gold and/or Silver have been also observed in quartz (Figure 9(d)).
3.3. Whole Rock Geochemistry: Classification and Geodynamic Settings
3.3.1. Volcanics Rocks
The composition of the analyzed samples follows that of mafic rocks characterized by relatively low SiO2 contents (43.35 wt% to 55.5 wt%). MgO content varies
![]()
Figure 9. (a) Amphibole sections stretched along the deformation direction; (b) Chlorite and biotite range in the amphibolite (LN); (c) Impregnation of calcite on rock (LP); (d) Ag-Au grain in quartz.
from 3.82 wt% to 22.42 wt% and the Al2O3 content varies from 6.39 to 18.85 wt.%. The results also highlighted a high content of FeOt (7.17 wt% - 12.54 wt%) and CaO (3.64 wt% - 13.41 wt%), while the contents of MnO (0.13 wt% - 0.18 wt%), P2O5 (0.04 wt% - 0.11 wt%) and TiO2 (0.41 wt% - 0.76 wt%) are considerably lower. The studied rocks show high contents of Sr (35 - 310.8 ppm), Zr (15.9 - 84.9 ppm), Ni (16.9 - 688.6 ppm), and Cr (29.9 - 1550 ppm). Based on the diagram of [29], the rock samples have predominantly basaltic and basaltic-andesite rock compositions (Figure 10).
On the Zr-MgO diagram, all the samples are discriminated to be ortho-amphibolite (Figure 11(a)). The studied rocks have a calc-alkaline affinity (Figure 11(b)) and they exhibited arc-basalt setting (Figure 11(c)). Trace element contents of the studied samples were also used to determine the possible tectonic setting of the rocks. The distribution of our samples on the triangular diagram of [30], shows that our amphibolites are predominantly grouped in the volcanic-arc basalts setting of the orogenic domain (Figure 11(d)).
3.3.2. Granitoïds
The studied granitoid samples consist of SiO2 (57.66 wt% - 69.51 wt%), 0.59 wt% - 4.08 wt% of K2O, and significant concentrations of Al2O3 (15.77-19.18 wt.%), Na2O (0.39 wt% - 5.29 wt%), FeOt (2.76 wt% - 7.8 wt%) and MgO (0.81 wt% - 2.82 wt%). As for the trace metal content, these rocks showed high levels of Sr (14 to 341.1 ppm), Rb (28.3 to 341.9 ppm) and Y (up to 26.6 ppm), leading to a low to moderate Sr/Y ratio (2.09 to 51.68 ppm). According to the geochemical classification diagram of (Middlemost, 1994), our granitoid classifies as diorite
![]()
Figure 10. Total alkaline-silica (TAS: [29]) diagram for the volcanic rocks of the Etéké gold district.
![]()
Figure 11. (a) Zr vs. MgO [31]; (b) Th vs. Co [32]; (c) Th–Hf/3–Ta [33] and (d) La-Y-Nb [30] diagrams for the amphibolite samples.
and granodiorite when using the classification of Middlemost (Figure 12).
The granitoids samples, characterized by a diorite to granodiorite geochemical composition, are predominantly calc-alkaline and moderately to highly potassic rocks (Figure 13(a)). The nature of these rocks is also found to be metalliferous to peraluminous (Figure 13(b)). Furthermore, the projection of these rocks in the diagram of (Pearce et al., 1984) indicates that all the studied granitoid samples are classified in the volcanic arc granitoid (VAG) domain (Figure 13(c), Figure 13(d)). These geochemical characteristics indicate that the Etéké granitoids could be derived from the partial melting of hydrated basaltic crust in subduction zones and from garnet and/or amphibole residues [34] [35] [36] [37].
4. Conclusions
The geological history of Gabon provides information on the formation of several strategic natural resources with valuable economic interest (gold, manganese, iron, etc.). Gabonese gold, aim of this study, is produced intermittently and
![]()
Figure 12. Classification diagram [38] of the granitoïds from Etéké gold district.
![]()
Figure 13. K2O vs. SiO2 diagram [39]; (b) A/CNK–A/NK diagram [40] and ((c), (d)) Tectonic discrimination diagrams [41] for the Etéké granitoïd samples.
generated mainly from the Etéké region.
In the Eteké region, the gold mineralization is linked to the Eburnean orogeny. The Etéké gold districts display considerably significant alteration. The most common alterations are carboniferous and silicified impregnations. Otherwise, the ferriferous oxidations prove to be the major characteristic in certain deposits. Five primary sites have been characterized in the Etéké region:
· Dango zone: it is well-known by a SIM whose origin is synchronous with the deformation, on the basis of the tectonic fabric which is permeated in the silicification. this deposit corresponds to an orogenic mineralization system with significant silicification during shearing.
· Dondo-Mobi zone: it represents a characteristic orogenic system formed by weak tectonic shortening and a usual geometry of sub-horizontal stress veins and more inclined shear veins. Random biotite is the hydrothermal alteration mineral as the mineralization was emplaced at slightly elevated temperatures.
· Ovala zone: It exhibits the opposite characteristics genetically. For example, the predominant element, muscovite shale, is not associated with gold mineralization. On the other hand, the high silicification, the pyritization in sterile zones and gold-bearing zones, parallel to the bedding and essentially the occurrence of kyanite porphyroblasts fit perfectly into a model of metavolcanogenic gold mineralization.
· Massima deposit: it is typically part of volcanogenic systems, this results in the disseminations of pyrites in volcano-sediments separated by late sills and also the unit of mineralized basalt with silicified gold zones with black chlorite.
· Moukanda prospect: it is characterized by the biotite abundance (source of the fabric), the upper green schist facies which is belonged to the metamorphism and some parts downgraded to chlorite
Petrographic and geochemical characterization of amphibolite and granitoid samples from the Etéké gold district were used to define their petrogenesis and tectonic frameworks. Therefore, we can retain:
· The studied volcanic rocks show geochemical compositions of basalt to basaltic andesite;
· The geochemical signature indicates that the Eteké amphibolite are basaltic arc and probably originate from a subduction zone;
· The studied granitoids show a geochemical evolution from diorite to grano-diorite and would come from the partial melting of the hydrated basaltic crust in the subduction zones.